Disable https://wiki.mozilla.org/Advocacy/heartbeat Firefox 37+ feature

in browser sessions started by FirefoxDriver.

According to the documentation,
"Every day a random subset of users are offered a rating widget",
which doesn't sound great for automated tests. We have not really
recorded any harmful effects (such as flakiness) of this feature
on tests, but I thought this change should keep everyone safe from that.

This is the only way to disable the feature that we found so far.

Author: sevaseva

Diff for: javascript/firefox-driver/webdriver.json

@@ -12,6 +12,7 @@
     "browser.safebrowsing.enabled": false,
     "browser.safebrowsing.malware.enabled": false,
     "browser.search.update": false,
+    "browser.selfsupport.url" : "",
     "browser.sessionstore.resume_from_crash": false,
     "browser.shell.checkDefaultBrowser": false,
     "browser.tabs.warnOnClose": false,